Heretic keys

The Heretic keys are the three color-coded key objects found in Heretic which allow the player's progress through the game's levels to be controlled. While Heretic's keys are more similar to Doom's in their behavior than to the Hexen keys, there are still a few differences:
 * There is only one type for each key color, as opposed to Doom's system of like-colored key cards and skull keys.
 * The colors are yellow, green and blue instead of yellow, red and blue.
 * Vanilla levels are always designed so that keys are obtained in the order yellow, then green, then blue. This convention is not necessarily respected by mods.
 * On the easiest skill level, keys appear on the automap.
 * Locked doors are drawn with their key color on the automap.
 * Almost all levels are consciously designed to use raise-and-stay-open door types for locked doors. This has a significant positive impact on cooperative multiplayer, ensuring that players who die do not need to pick up every key in the map again to keep playing.

Data
The automap color corresponds to the PLAYPAL palette index used to draw locked doors and, if appropriate, keys themselves in the automap. For convenience, the RGB value for that index in the first palette is also provided.

Appearance statistics
The IWAD contains the following numbers of keys:

Key gizmos
Unlike in Doom where special textures are used on door jambs to indicate the color of key needed to open that door, Heretic relies on special objects internally referred to as "key gizmos."

In game, these appear as carved stone statues in the form of a winged and horned creature with a coiled serpent tail, similar in design to the Ssair dragons from Shadowcaster, which each hold up an orb of the same color as the respective key needed to open the door. Most locked doors are flanked by two such statues, though they occasionally occur alone or not at all.

Trivia

 * In the manual, the yellow key is called gold.